Fuel cells have recently received much attention as new electric generators. In the near future, the fuel cells will substitute for the existing electric generators as either automobile batteries, power sources for electric generation or portable electric sources.
A polymer electrolyte fuel cell is a kind of a direct current generator converting the chemical energy of fuel directly to electric energy by electrochemical reaction. It comprises a continuous stack complex equipped with membrane-electrode assemblies which are the heart of the fuel cell and bipolar plates which serves to collect generated electricity and to supply fuel. The membrane-electrode assembly refers to an assembly comprising: an electrode where electrochemical catalytic reaction occurs between fuel (aqueous methanol solution or hydrogen) and air; and a polymer membrane where the transfer of hydrogen ions occurs.
Meanwhile, all electrochemical reactions consist of two individual reactions, i.e., oxidation reaction occurring at an anode(fuel electrode), and reduction reaction occurring at a cathode(air electrode), in which the two electrodes are separated from each other by a polymer electrolyte membrane. In a direct methanol fuel cell, methanol and water in place of hydrogen are supplied to the anode, and hydrogen ions produced in an oxidation process of methanol are transferred to the cathode through the polymer electrolyte membrane and generates electricity by reduction reaction with oxygen supplied to the cathode. Such reactions are as follows:
anode (fuel electrode): CH3OH+H2O→CO2+6H++6e−
cathode(air electrode): 3/2O2+5H++6e→3H2O
Overall reaction: CH3OH+ 3/2O2→CO2+3H2O
The electrode of the direct methanol fuel cell is typically a diffusion electrode. The electrode consists of two layers, a gas diffusion layer (electrode support layer) and an active layer. The gas diffusion layer serves as a support and to diffuse fuel, and is made of carbon paper or carbon cloth. The active layer is adjacent to the polymer electrolyte membrane to cause substantial electrochemical reaction and is made of either a platinum catalyst particle dispersed in a carbon particle or platinum or alloy black. The electrochemical reaction occurs at a three-phase interfacial zone in which fuel diffused from the gas diffusion layer is exposed to the interface between the electrolyte membrane and the platinum catalyst particle of the active layer. Thus, it is important for the improvement of performance to enlarge the area of the three-phase reaction zone, which is available in the electrochemical reaction, and to place the platinum catalyst in the three-phase reaction zone to the maximum possible extent. However, unlike a liquid electrolyte, a depth to which the solid polymer electrolyte membrane can be impregnated into the electrode is limited to 10 μm, so that the area of the three-phase reaction zone, which can be enlarged, is limited, and only a portion of the platinum catalyst, which is exposed to the three-phase reaction zone, can participate in the electrochemical reaction in the fuel cell. Accordingly, in order to increase the power density of the fuel cell, an electrode structure is required in which the area of the three-phase reaction zone is maximized and the maximum possible amount of platinum is placed in the active layer which is in contact with the electrolyte.
In the initial development stage of the direct methanol fuel cells, an electrode was used which had been prepared by adding Pt-black particles onto carbon paper or carbon cloth used as a diffusion layer by a spray, etc., so as to form an active layer, and adhering the active layer to an electrolyte membrane by a hot-pressing process. However, the prior structure had problems in that the interfacial resistance between the active layer and the electrolyte membrane was high to make the structure inefficient, and a significant amount of the catalyst particles penetrated into the diffusion layer and thus did not participate in the reaction, indicating that the expensive noble catalyst was useless.
In attempts to solve such problems, methods of forming a catalytic layer directly on an electrolyte membrane as in a decal process (U.S. Pat. No. 6,391,486) and a sputter deposition process (U.S. Pat. No. 6,171,721) were proposed.
However, the decal process is one comprising forming an active layer separately and then laminating the active layer with an electrolyte membrane, but requires a higher temperature than the glass transition temperature of the electrolyte membrane upon the laminating step, thus requiring separate pretreatment which makes the process complex. Another problem is that the transfer of the separately formed active layer is not properly performed.
In the sputter deposition process, the efficiency of a catalyst can be increased, but a thin film is formed at a thickness of more than 1 μm due to the crystalline nature of the catalyst, thus preventing the transport of cations. Accordingly, only a very small amount of the catalyst will inevitably be used, resulting in a reduction in power density. Also, the sputter deposition process has problems in that a higher power density than a given level can not obtained, and a high-vacuum region is used due to the characteristic of a semiconductor process, resulting in increases in production cost and time, which renders the process unsuitable for mass production.
While the above-described coating methods have their own advantages, they have a serious disadvantage in that it is difficult to form a stable interface between a solid polymer electrolyte membrane and nanosized catalyst particles.